The search for high energy density batteries has motivated research in lithium-air batteries. Catalysts have been shown to improve both the battery capacity and the recyclability of these batteries when used in cathodes.
High energy density batteries have garnered much attention in recent years due to their demand in electric vehicles. Lithium oxygen (Li—O2) batteries have nearly 10 times the theoretical specific energy of common lithium-ion batteries and in that respect have been regarded as the batteries of the future. A typical Li—O2 battery comprises a Li anode, a porous cathode open to oxygen, and a Li+ ion conducting electrolyte separating the electrodes. A Li—O2 battery stores energy via a simple electrochemical reaction (2Li+O2↔Li2O2) in which Li2O2 is deposited on the surface of the cathode via a forward reaction (oxygen reduction reaction, ORR) during discharge and a backward reaction (oxygen evolution reaction, OER) takes place during charging to decompose Li2O2 on the surface of cathode. Since the main discharge product (Li2O2) and other discharge/charge byproducts in Li—O2 batteries are electrically insulating and not soluble in electrolytes, the structure and electronic conductivity of cathode materials have been critical factors in determining the limiting capacity of Li—O2 batteries. Carbonaceous materials such as carbon nanoparticles, carbon nanofibers, carbon nanotubes, graphene platelets, and other forms of carbons have been commonly used as cathode materials in Li—O2 batteries. Among carbon-based materials, carbon nanotubes (CNTs) have been widely used in Li—O2 cathodes due to their high specific surface area, good chemical stability, high electrical conductivity, and large accessibility of active sites. CNT (single-walled) have been used as cathode materials in Li—O2 batteries and shown discharge specific capacities as high as 2540 mAh·g−1, which were obtained at a 0.1 mA·cm−2 discharge current density.
Although many research studies have been done to improve the performance metrics of Li—O2 batteries, they are still in their early stages and many technical challenges have to be addressed before their practical applications.
The most common problems impeding the development of Li—O2 batteries have been low rate capability, poor recyclability, and low round-trip efficiency. All of these issues are originally stemmed from sluggish kinetics and the irreversible characteristic of the OER and ORR reactions which causes high overpotentials in the discharging/charging process. Hence, increasing the efficiency of OER/ORR reactions and minimizing the overpotentials during the discharging/charging process have been regarded as a meaningful approaches to overcome the aforementioned problems in Li—O2 batteries.
Various additives have been explored to remedy this problem including the use of redox mediators. Redox mediators minimize charge polarization by acting as charge carriers between the cathode and Li2O2 surface. Alternatively, different noble metals and metal oxide catalysts have also been integrated in the cathodes of Li—O2 batteries. The catalyst may influence the performance of Li—O2 batteries by destabilizing the oxidizing species which decreases the charging overpotential. They may also increase the surface active sites and facilitate charge transport from oxidized reactants to the electrode which can also lead to formation of nanocrystalline Li2O2. However, it has been recently shown that the catalyst on the oxygen cathode in Li—O2 batteries is easily deactivated due to continuous accumulation of discharge and charge products upon cycling. It also has been reported that coarsening and agglomeration of catalyst upon charging/discharging reduces the efficiency of catalyst in Li—O2 batteries. Platinum (Pt) and palladium (Pd) catalysts have been reported to promote Li2O2 oxidation at low potentials but also cause electrolyte decomposition resulting in the formation of Li2CO3 and thus deactivating the catalysts.